1. Technical Field
The present invention relates to a thermoelectric conversion module that converts thermal energy to electric energy, and relates to a production method therefor. The thermoelectric conversion module has a bonding portion between a thermoelectric conversion element and an electrode, and thermal stress may occur in the bonding portion. Therefore, specifically, the present invention relates to a thermoelectric conversion module with a function to decrease the thermal stress, and relates to a production technique therefor.
2. Background Art
A power generation system provided with a thermoelectric conversion module using thermoelectric conversion elements directly generates electricity. This power generation system has a simple structure and does not have a movable part, thereby having high reliability and facilitating maintenance thereof. However, the power generation system has low power density and low energy conversion efficiency. Due to this, this power generation system has been developed only for special uses at low output scale, such as for use in space. However, in view of recent environmental issues, this power generation system is expected to be useful as an environmental protection measure. For example, this power generation system is anticipated to be useful as a small-scale distributed power generation system that uses an exhaust heat source of a waste incinerator, a cogeneration system, etc. This power generation system is also anticipated to be useful as an automobile power generation system that uses heat in exhaust gas of automobiles. Consequently, reduction in the cost of this power generation and improvement in durability of a thermoelectric conversion module system are required in this power generation system.
A thermoelectric conversion module is shown in FIG. 1. As shown in FIG. 1, the thermoelectric conversion module is constructed by stacking an electrode 2 on each side of a thermoelectric conversion element 1. Moreover, each of a cooling duct 4 and a heating duct 5 is stacked on the rest side of each electrode 2 via an electric insulating layer 3. The electrode 2 may be made of copper, and the electric insulating layer 3 may be made of mica. In this thermoelectric conversion module, by sending air to the cooling duct 4 and by supplying high-temperature exhaust gas to the heating duct 5, a temperature difference is generated between the two ends of the thermoelectric conversion element 1. The temperature difference generates thermoelectric power in the thermoelectric conversion element 1, whereby direct current is obtained from the electrode 2. Such a thermoelectric conversion module is disclosed in Japanese Patent Application of Laid-open No. 9-293906, for example.
In general, the thermoelectric conversion module is produced by pressing and bonding the thermoelectric conversion element and the electrodes, or by bonding them with a soldering material. As described above, the thermoelectric conversion module generates power based on the thermoelectric power. The thermoelectric power occurs by the temperature difference between the two ends of the thermoelectric conversion element. Therefore, when the temperature difference between the two ends of the thermoelectric conversion element is larger, the thermoelectric power is increased, and a greater amount of electricity is generated. In order to increase the temperature difference between the two ends of the thermoelectric conversion element, the temperature of the cooling side (cooling duct 4) may be decreased, but a special device is required, which is not preferable. Accordingly, usually the temperature of the heating side (heating duct 5) is increased to not more than an upper temperature limit of the thermoelectric conversion element.
The thermoelectric conversion element and the electrode at the heating side of the thermoelectric conversion module have a bonding portion therebetween. The thermoelectric conversion element does not greatly expand with the heat, but the electrode greatly expands with the heat. Therefore, in the bonding portion, there is a difference in the amounts of the thermal expansion of the thermoelectric conversion element and the electrode. As a result, the bonding portion receives stress due to the difference in the amounts of the thermal expansion. Accordingly, when the temperature of the heating side of the thermoelectric conversion element is increased so as to increase the amount of power generation, large thermal stress occurs in the bonding portion by the difference in the amounts of the thermal expansion. The large thermal stress easily causes fracture at the bonding portion between the thermoelectric conversion element and the electrode and its vicinity.
The thermoelectric conversion element and the electrode may be bonded with a soft soldering material. In this case, if the temperature of the heating side is set to be not less than the melt temperature of the soft soldering material, the soft soldering material is melted and leaks. Therefore, in this kind of thermoelectric conversion module, the temperature of the heating side is limited, and the amount of the power generation is limited.